Peanut diseases surge in areas of excessive rainfall

With some areas of Alabama receiving as much as 10 to 30 inches of rainfall in a short period of time, controlling diseases in peanuts is becoming a major concern for producers.

By Austin Hagan, Alabama Cooperative Extension System | Aug 13, 2013

In recent weeks, peanut production areas across Alabama have received a minimum of 10 to upwards of 30 inches of rainfall, which has prevented many producers from making timely leaf spot fungicide applications or has accelerated the erosion of fungicide residues on peanut foliage.

As a result, early leaf spot has started to surge in a good many peanut fields with late leaf spot and peanut rust likely to follow in coming weeks. In addition, several peanut varieties, particularly Georgia-09B, are susceptible to leaf spot diseases. So, conditions favor damaging leaf spot outbreaks, especially if frequent showers continue through the remainder of the summer.

Peanut producers need to scout their fields weekly to determine whether early or possibly late leaf spot has gotten a head start. When leaf spotting is seen, an application of Headline at a minimum of 9 fluid ounces alone or in combination with a triazole fungicide such a either a generic tebuconazole (i.e. Muscle, Tebustar, Orius, etc.) at 7.2 fluid ounces per acre, Alto at 5.5 fluid ounces per acre, or Topguard at 7 fluid ounces per acre needs to be made.

If needed, follow with another recommended leaf spot fungicide before making a second Headline application and then continuing with their season long disease control program up to 14 days before digging.

In fields at risk from leaf spot diseases and rust (mainly Baldwin and Mobile counties), shorten the interval between fungicide applications to 10 to no more than 14 days. When using a chlorothalonil fungicide (Bravo Weather Stik, Bravo Ultrex, Echo, Equus, Chloronil, etc.) over the next few weeks, add a generic tebuconazole (i.e. Muscle, Tebustar, Orius, etc.) at 7.2 fluid ounces per acre, Alto at 5.5 fluid ounces per acre, Topguard at 7 fluid ounces per acre, Quash, or propiconazole (Tilt, Bumper, etc.) to the tank mixture to get some "kick back" activity against new infections by early and late leaf spot fungi.

One caution: generic tebuconazole fungicides are not rainfast. Wash-off occurs when a rain shower occurs within 24 hours of an application of these fungicides. If that happens, a follow up fungicide application needs to be made within seven days. Better leaf retention will be obtained with generic tebuconazole fungicide by tank mixing them with a chlorothalonil fungicide. In contrast, Headline and possibly other strobilurin fungicides like Abound and Evito tend to have good to excellent rainfastness on peanut foliage.

Peanut rust will likely be a serious threat to peanuts grown in Baldwin and Mobile counties as well as Escambia, Covington, Geneva and Houston counties. As noted above, peanut producers in these counties are strongly advised to scout their peanuts weekly beginning in August for rust hot spots.

Nearly all fungicides that have good leaf spot activity will also control rust. Other than using a recommended fungicide, the best strategy for controlling rust involves shortening fungicide spray intervals to 10 days. Once rust gets rolling in peanuts, it's very hard to stop.

Stem rot off to fast start

Stem rot, or white mold, has also gotten off to a fast start in peanuts. It’s much more of a problem in the traditional peanut production area in southeast Alabama but is showing up in field trials at the Gulf Coast Research and Extension Center in Fairhope. I

Incidence of stem rot is typically much higher the more frequently peanuts are cropped in individual fields but there are a few fields that historically have high levels of this disease regardless of cropping history.

Most widely planted runner peanut varieties, except maybe Florida 07 and Georgia-07W, are fairly susceptible to stem rot. Convoy at rates from 20 to 32 fluid ounces per acre tends to have a slight edge in performance in high stem rot pressure situations. Otherwise, Fontelis, Provost, Abound alone or tank mixed with Alto, and Priaxor when applied at recommended rates for stem rot control also are effective against this disease.

Headline has good stem rot activity but must be applied at night when the leaves are folded to get best results. Other strobilurin fungicides like Abound would also benefit from night applications. The generic tebuconazole fungicides as well as Evito, Evito T, Topguard and Quash also have stem rot activity and would be more suitable in lower pressure situations.

While thrips damage levels were high in a lot of peanut fields, TSWV pressure is fairly low and the disease is unlikely to cause any yield losses.

Alto registered in peanuts

As I mentioned above, Alto fungicide, which contains the active ingredient cyproconazole, was recently registered for use in peanut for the control of leaf spot diseases and rust when applied twice at 5.5 fluid ounces per acre or tank-mixed at the 5.5 fluid ounces per acre rate with Abound 2SC at 12 to 24 fluid ounces per acre approximately 60 and 90 days after planting for the control of the above diseases and stem rot (white mold) as part of a season long disease management program.

Years ago, Alto was extensively screened on peanuts and proved to have excellent activity against leaf spot diseases and stem rot. Alto is a triazole fungicide and in areas where triazole-resistance in leaf spot fungi has been an issue, it must be tank-mixed with 0.75 to 1.0 pint per acre of a chlorothalonil fungicide (Bravo Weather Stik, Bravo Ultrex, Echo, Equus, Chloronil, etc.) to insure effective leaf spot control.